1. Field of the Invention
The present invention generally relates to a power steering system for vehicles. More particularly, the invention relates to an electric power steering system for vehicles which produces ausiliary torque for steering by means of a steering serve unit using an electric motor.
2. Description of Relevant Art
In view of the problems of hydraulic type power steering system such as the structure thereof is complicated, recent years have proposed a variety of electric type power sttering systems for vehicles.
Exemplarily, in Japanese Patent Application Lay-Open Print No. 59-70257, laid open on Apr. 20, 1984, there is disclosed an electric power steering system for vehicles of an analogue control type.
This electric power steering system for vehicles includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected through a universal joint with the input shaft and through a gear mechanism of a rack and pinion type with a tie rod of a steered wheel. An electric motor supplies auxiliary torque through a reduction gearing to the output shaft. A torque detection mechanism is disposed on the input shaft to detect steering torque acting on the input shaft, and a driving control circuit produces, based on a detection signal from the torque detection mechanism, a torque magnitude signal and a torque direction signal representing the magnitude and the direction of the steering torque acting on the input shaft, respectively. The driving control circuit feeds the electric motor with an armature current in proportional relation to the quantity to the torque magnitude signal and in accordance to the direction of conduction with the torque direction signal.
The torque detection mechanism consists of a strain gauge sensor.
With such an arrangement, when the steering wheel is operated, the output shaft is applied with adequate auxiliary torque from the electric motor, so that the steering operating is facilitated.
In the electric power steering system described, however, the driving of the electric motor is controlled in such a manner that auxiliary torque, the magnitude of which is substantially proportional to that of the steering torque acting on the input shaft, is applied to the output shaft, irrespective of the speed of rotating operation of the steering wheel.
In the electric power steering system described, therefore, there might exist such a possibility that the steering speed of the steering wheel and the rotation speed of the electric motor may fail to be always kept in a well-matched relationship to each other.
In recent years, there is a tendency to utilize micro-computer systems, which fundamentally are for processing digital signals, as control devices of various working systems, since they have the advantage that complicated control functions can be achieved with a relatively simple system constitution.
It is thus desirous to employ a micro-computer system as a control device for such a steering system as described.
However, in general, the micro-computer system is unable to simultaneously read many input signals, and process operations thereof are sequentially performed in accordance with a system clock pulse. As a result, there is a restriction or problem such that it needs a predetermined process time to attain, with a micro-computer system, various control function similar to those of an analogue control circuit in, for example, the conventional power steering system described.
In this respect, in the case of a micro-computer system responsible for a feedback control, such a problem is remarkable due to the necessity of repeating a feedback loop so many times.
For such reasons, in the cse where a micro-computer system is utilized as a control device of an electric power steering system, there might be a possibility such that the control of an electric motor adapted to produce auxiliary torque may fail to always sufficiently follow up the speed of steering operation.
The present invention has been achieved to effectively solve the problems of conventional electric power steering systems as described. Particularly, the invention has been achieved to overcome, even in the case where to solve such problems a micro-computer system is utilized as a control device, the aforementioned restriction or problem attendant thereon.